The present invention relates to a method for inspecting a zone for detecting, and optionally for counting, one or more objects, inclusions and/or formations in this zone. It also relates to a system implementing such a method.
The field of the invention is the field of detecting objects, inclusions, formations and, more generally, elements located in a zone or a region, termed the zone or region of interest, and more particularly of detecting the position of these elements and counting these elements. The invention can more particularly be used in the field of detecting underground or underwater objects or formations, for example in order to detect anti-personnel mines, and the field of medical imaging. In the latter field, the term object denotes an element present in the body of a person.
Different methods currently exist for inspecting a zone of interest in order to locate and count elements present in this zone.
These methods include the methods termed time-reversal imaging methods. These methods consist of emitting an incident wave, for example an acoustic wave, towards a zone of interest from at least two emission points. A set of receivers, arranged at a certain distance from the zone of interest, for example at the boundary of the zone of interest, measures the diffracted/reflected portion of the incident wave, called the diffracted wave. The diffracted wave is then reversed in the time domain in order to determine, by numerical calculation or analytical formula, a wave, termed the reversed wave. For each emission point, a time integration of the product of the incident wave and the reversed wave according to a known relationship makes it possible to determine a signal, termed the detection signal, that makes it possible to learn whether there is an element in the zone of interest and the position of the element in the zone of interest. The results of the detection can be improved by adding the detection signal obtained for one emission point to the detection signal obtained for another emission point. This method is known by the name Reverse Time Migration (RTM), see Claerbout (1985, Imaging the Earth's interior).
For more details on time-reversal detection methods see the articles:                “Time Reversed Absorbing Condition: Application to inverse problem”, http://hal.archives-ouvertes.fr/hal-00491912; and        “Time Reversed Absorbing Condition in the Partial Aperture Case”, http://hal.archives-ouvertes.fr/hal-00581291.        
The currently known methods make it possible to carry out a detection of elements when the distance separating two elements is greater than or equal to the half wavelength of the incident wave.
An aim of the present invention is to overcome the aforementioned drawback.
Another aim of the present invention is to propose a method and a system for inspecting a zone of interest that are more accurate than the known methods and systems.
Finally, another aim of the present invention is to propose a method and a system for inspecting a zone of interest that make it possible to detect two elements separated by a distance smaller than the half wavelength of the wave used.